While loops execute code repeatedly as long as a condition is true. They’re useful when you don’t know in advance how many iterations you need.

Basic Syntax

count = 0
while count < 5:
    print(count)
    count += 1

# Output: 0, 1, 2, 3, 4

Comparison with For Loops

# For loop - when you know the iterations
for i in range(5):
    print(i)

# While loop - when condition-based
user_input = ""
while user_input != "quit":
    user_input = input("Enter command: ")
    print(f"You entered: {user_input}")

Using break

Exit the loop early:

while True:
    user_input = input("Enter a number (or 'quit'): ")
    
    if user_input == "quit":
        break
    
    number = int(user_input)
    print(f"Square: {number ** 2}")

print("Goodbye!")

Using continue

Skip to the next iteration:

count = 0
while count < 10:
    count += 1
    
    if count % 2 == 0:
        continue  # Skip even numbers
    
    print(count)

# Output: 1, 3, 5, 7, 9

While-Else

The else block runs when the condition becomes false (not on break):

count = 0
while count < 3:
    print(count)
    count += 1
else:
    print("Loop completed normally")

# Output: 0, 1, 2, Loop completed normally

# With break - else doesn't run
count = 0
while count < 3:
    if count == 1:
        break
    print(count)
    count += 1
else:
    print("This won't print")

# Output: 0

Common Patterns

Countdown Timer

import time

countdown = 5
while countdown > 0:
    print(countdown)
    countdown -= 1
    time.sleep(1)
print("Blast off!")

Validating Input

while True:
    age = input("Enter your age: ")
    
    if age.isdigit() and int(age) > 0:
        age = int(age)
        break
    
    print("Please enter a valid positive number")

print(f"Your age is {age}")

Processing a List

items = ["apple", "banana", "cherry"]

while items:  # While list is not empty
    item = items.pop()
    print(f"Processing: {item}")

# Output: cherry, banana, apple

Finding Values

import random

target = 7
attempts = 0

while True:
    guess = random.randint(1, 10)
    attempts += 1
    
    if guess == target:
        print(f"Found {target} in {attempts} attempts!")
        break

Avoiding Infinite Loops

# Bad - infinite loop (missing increment)
# count = 0
# while count < 5:
#     print(count)  # count never changes!

# Good - always update the condition variable
count = 0
while count < 5:
    print(count)
    count += 1  # Don't forget this!

# Safe pattern with a maximum iterations
max_iterations = 1000
iterations = 0

while some_condition and iterations < max_iterations:
    # Do work
    iterations += 1

if iterations >= max_iterations:
    print("Warning: Maximum iterations reached")

Nested While Loops

outer = 0
while outer < 3:
    inner = 0
    while inner < 3:
        print(f"({outer}, {inner})")
        inner += 1
    outer += 1

# Output: (0,0), (0,1), (0,2), (1,0), (1,1), (1,2), (2,0), (2,1), (2,2)

Summary

Use while condition: for condition-based loops
Use break to exit early
Use continue to skip iterations
Use else for code that runs when loop completes normally
Always ensure the condition will eventually become false
Use a safety counter for potentially infinite loops

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